Monday, 30 November 2009

While dismantling my extruder for a small mod I accidentally discovered that the worm pulley has so much grip that it will still extrude PLA with the pressure roller removed. It is hard to see on this low quality video but it was extruding 0.4mm filament at 32mm/s.

I will still run it with a roller because it helps to guide it into the tube when self feeding to start a new filament. I also expect softer plastics would need it.

BTW, I have stopped using Vimeo and gone back to YouTube because they added an artificial processing delay unless you pay.

As the worm gear is about twice the diameter of the threaded pulley the axial force on the motor is about half the force required to push the filament, i.e. a few kilograms. After making a few objects it cracked along the layer where the bearing housing rises out of the flat motor mount.

I designed a new bracket but I was back in a chicken and egg situation with no working extruder to print it. As Erik pointed out you need a Robin Hood / Friar Tuck strategy of having two machines so that one can make replacement parts for the other. I must get my Darwin up and running!

In the meantime I cobbled it back together with some random bits of metal, some tiny G-clamps and tie wraps: -

I made some of the new bracket thicker where I could: 8mm instead of 5mm, which should be ~2.5 times stronger. I also added some ribs and extruded it at 10°C higher temperature.

This one seems solid as a rock, but it did warp a little more. The stronger you make something the more it warps.

Wednesday, 4 November 2009

I have settled on using vitreous enamel resistors embedded in an aluminium block for the heater. I think they are the easiest heater to make and likely to be the most durable. They also work fine with simple bang-bang control, whereas it would appear that the Nichrome and Kapton version requires PID.

One of the aims of my new design is to reduce the amount of molten plastic to minimise ooze. Also less molten plastic means less viscous drag. I also wanted to reduce the thermal mass (to reduce the warm up time) and completely cover the hot part with insulation to allow a fan to blow on the work-piece without cooling the nozzle.

To achieve these aims I switched to a smaller resistor (same resistance but less wattage) and mounted it horizontally rather than vertically. There is some risk that the resistor may fail but I think as long as it has good thermal contact with the aluminium block, so that its outside temperature is less than 240C, then I have a good chance it will last.

The smaller resistor also means a much smaller surface area so less heat is lost. T0 keep the molten filament path as short as possible I combined the heater and the nozzle and made it from one piece of aluminium. That also gives very good thermal coupling between the nozzle tip, the melt chamber, the heater and the thermistor.

I turned it out of a block of aluminium using my manual lathe and a four jaw chuck, but I think I could also mill it out of 12mm bar using HydraRaptor.

A feature that I have used on my previous extruders is to cover as much of the nozzle as possible with PTFE. That stops the filament sticking so that it can be wiped off reliably with a brush. It also insulates the nozzle.

My previous nozzle cap implementations have been turned from PTFE rod. The downside of that is that the working face, that has been cut and faced on the lathe, is not as smooth and slippery as the original stock.

To cover the face of this version I used a 3mm sheet of PTFE so it has the original shiny surface.

Normally PTFE is too slippery to glue so my original plan was to screw it on with some tiny countersunk screws. However, the sheet I bought was etched on the back to allow it to be glued, so I stuck it on with RTV silicone adhesive sold for gluing hinges onto glass oven doors.

To insulate the rest of the heater I milled a cover out of a slice of 25mm PTFE rod.

I normally stick items to be milled onto the back of a floor laminate off-cut using stencil mount spray. I didn't think that was going to work with a PTFE cylindrical slice that is only a little bigger than the finished item. Instead I milled a hole in a piece of 6mm acrylic sheet that was already stuck down with stencil mount. The hole was slightly smaller than the PTFE so I faced it and chamfered it on the lathe and then hammered it in.

I roughed the shape with a 1/8" end mill and then sharpened the internal corners and cut the slots for the resistor leads with a 1mm end mill. I tried to mill the whole thing with a 1mm bit but it snapped due to a build up of burr in the deep pocket. On reflection it was silly to expect to be able to mill deep pockets with a 1mm bit and of course it is much faster to rough it with a bigger bit.

I used my normal technique of taking 0.1mm depth cuts at 16mm. That allows me to mill plastic with no coolant, but I expect I could have made much deeper cuts in PTFE. It mills very nicely, probably because it is soft and has a high melting point and low friction.

I haven't done any milling for a long time so for anybody new to my blog here is my the milling set-up: -

It is simply a Minicraft drill with some very sturdy mounts. The spindle controller I made originally would need its micro replaced as the one I used has a bug in its I2C interface. Instead I just connected it to the spare high current output on my new extruder controller.

The remaining part of the extruder is the stainless steel insulator.

I made the transition zone shorter than the last one I made because I wanted all of the inside of the transition to be tapered. The aluminium sleeve carries away the heat from the cold end of the transition to an aluminium plate that forms the base of the extruder. That in turn carries the heat to the z-axis via an aluminium bracket. I used heatsink compound on the joints.

Here is a view of the bottom half of the extruder: -

And here is a cross section showing the internal details: -

So that was the plan, what could go wrong? Well everything really! The first problem was that the resistor shorted out to the aluminium block. The smaller resistor only has a thin layer of enamel over its wire. Normally I wrap aluminium foil round it to make it a tight fit. I didn't drill the hole big enough so it was a tight fit with only one layer and pushing it in abraded the enamel. The solution would be a bigger hole and more layers of foil, but I just glued it with Cerastil as a quick fix. Of course it only failed after I had fully assembled it and run some heat cycles so I had to strip it down again to fix it. Not easy once the wiring has been added.

The next problem is that it leaks. I think it is because I dropped the extruder when I was building it and bent the thin edge at the end of the stainless steel barrel. That forms the seal with the heater block, so even though I straightened it I think the seal is compromised. I keep tightening it and thinking it is fixed but after hours of operation plastic starts to appear at the bottom of the PTFE cover.

The other problem is that mostly it extrudes very well, I now do the outline at 16mm/s and the infill at 32mm/s, but sometimes the force needed to push the filament gets higher and causes the motor to skip steps, or the bracket to bend so far that the worm gear skips a tooth.

I have made several objects taking between one and two hours and it worked fine. Other times, mainly when I was making small test objects with Erik, it will completely jam. Actually it seems to jam when it is leaking badly, which implies the pressure of the molten plastic is much higher as well as the force to push the filament. The only explanation I can think of is there is an intermittent blockage of the nozzle exit. More investigation required.

Tuesday, 3 November 2009

Erik de Bruijn (RepRap evangelist) is in the UK at the moment visiting Salford and Nottingham universities to spread the word. Yesterday he came here to see HydraRaptor. We spent a very interesting afternoon and evening, swapping extruder ideas, comparing objects we had made, and doing a couple of very successful experiments.

The first was something I had been wanting to try for a long time, and that was reversing the extruder drive to stop ooze. My latest extruder (details to follow) has a much smaller melt chamber but still has significant ooze when extruding PLA. Erik is pursuing the Bowden extruder idea, which should benefit even more from reversing.

Because my machine is controlled by Python, rather than g-code, it is very easy to try out things like this. We hacked the code to instantaneously reverse for a short distance very quickly at the end of each filament run. After moving to the start of the next run it fast forwards the same distance that it reversed before resuming the normal flow rate.

I designed a simple test shape to allow the results to be compared. It is a 15mm square with four 5mm towers at each corner. I am not using Enrique's latest Skeinforge which I think would minimise the extruder moves in fresh air to just three per layer. This is with a very old version that does the four outlines and then returns to fill each of them in.

Plenty of hairy bits showing the ooze. These can be removed easily, but what is worse is the object will be missing that amount of plastic making it weaker. This can be extreme with a thin structure which is remote from other parts of the same object.

We tried reversing 1 mm at 8 times the extrusion speed to start with. That worked but was obviously more than was needed. We tried 0.25mm which was too little and settled on 0.5mm, although a lot of that is taken up by the motor bracket flexing. I need to make it stronger.

The result was no hair at all!

A very simple fix for a problem that has used a lot of my time in the last two years.

The second experiment was something Erik wanted to try. He has discovered that PLA is soluble in caustic soda, so potentially could be used as soluble support material for ABS. The question was: can we extrude ABS onto PLA and get it to stick well enough to resist warping?

We made a 5mm thick slab of PLA 20mm wide and 40mm long, 90% fill. On top of that we extruded a 30 x 10 x 20mm block of ABS with a 25% fill.

The ABS looks very glossy so I think it may have some PLA in it. Possibly we needed to flush it through for longer. The ABS block is also a bit scrappy. The reason was that the extruder was playing up. It was leaking plastic, hence the burnt bits and the stepper motor was skipping steps leaving a deficit of plastic. This extruder had never done ABS before and still has some teething problems, but it shows that ABS will bond to PLA well enough to stop it curling.

Next we extruded a block of PLA on top of the ABS.

That also bonded well. The messy bit at the join is because HydraRaptor did its normal circuit of the object that it normally does on the first layer but it was in mid air.

To see how well they were bonded we put the PLA base in a vice and attached a small g-clamp to the PLA block on top. The g-clamp was pulled with a strain gauge until the ABS came way from the base at about 8Kg. Interestingly the first layer outline of the ABS was left on the PLA. That was deposited at 215°C whereas the infill of the first layer was at 195°C. These are the values I use for depositing ABS onto a raft, so in an object layer on top of support it would be 240°C giving a stronger bond. See Erik's writeup and video here.

So PLA looks like a good candidate for supporting ABS. They bond well and PLA is very rigid to resist warping. It can be dissolved with drain cleaner but also I expect it would be easy to peel when softened in hot water.